Water level adjustment downstream of dams significantly impacts river regimes and flood control. However, due to constant strong scouring, our quantitative understanding of the characteristics of water level variations and their causes in the Chenglingji–Jiujiang Reach of the Yangtze River remains limited. Here, we analyzed the water level change trend via the Mann–Kendall method and analyzed geomorphic change and river resistance using 406 cross-sectional profiles as well as data on discharge and water levels from 1991 to 2022. Results showed that the critical conversion discharges (CCD) in the Chenglingji-Hankou Reach and the Hankou-Jiujiang Reach were approximately 35,000 and 30,000 m3/s, respectively, after the operation of the Three Gorges Dam. The water level exhibited an overall decline mainly due to river erosion when the discharge was lower than the CCD. The water level exhibited a nonsignificant upward trend mainly due to increased river resistance (7–20%) when the discharge was higher than the CCD. The obvious increase in the floodwater level in individual years was caused by the effect of downstream water level increase. Our findings further the understanding of downstream geomorphic response to dam operation and their impacts on water levels and have important implications for flood management in such rivers worldwide.

  • The critical conversion discharges of the water level changes in the Chenglingji-Hankou Reach and Hankou-Jiujiang Reach are approximately 35,000 and 30,000 m3/s, respectively.

  • The dominant cause of the water level decrease below or increase above the critical conversion discharge is river erosion or greater increase in resistance.

  • The obvious increase in the floodwater level in individual years was due to the more obvious increase of downstream water level.

Natural rivers often have a dynamic balance of erosion and deposition due to stable water and sediment conditions and riverbed boundaries over many years (Williams & Wolman 1984). However, the natural balance has been disrupted by increasing anthropogenic stresses on rivers, especially the construction of dams, with >45,000 large dams (heights >15 m) distributed across 140 countries (Ma et al. 2022; Syvitski et al. 2022). Therefore, geomorphological and hydrological changes and their influences on rivers under different conditions (e.g., natural status, anthropogenic stresses, climate change) have always been important topics in the fields of river geomorphology, river engineering, and river ecology (Schmidt & Wilcock 2008; Gao et al. 2021; Li et al. 2021).

The construction of large dams changes the spatial and temporal distributions of water and sediment downstream of dams, which decreases the original stability of the basin. Accordingly, the water level downstream of a dam changes, affecting the river regime, water supply, navigation, flood control, and ecology (Surian & Rinaldi 2003; Boulange et al. 2021; Hu et al. 2022b). However, the variations in the water level in the lower reaches of dams in different rivers greatly differ. For example, after the completion of the Aswan Dam of the Nile River (Saad 2002), the Glen Canyon Dam of the Colorado River (Shields et al. 2000), and the Fort Peck Dam of the Missouri River (Topping et al. 2003), the downstream channels generally experienced a decrease in the water level and a decrease in the water surface gradient due to cutting of the riverbed. After the Danjiangkou Reservoir of the Hanjiang River was impounded, the middle and low water levels at the downstream hydrological stations decreased, but the high water level increased by approximately 1.47 m from 2016 to 2017 due to the partial construction of the Huangzhuang–Datong Reach (Xiao et al. 2018; Huang et al. 2022). After the Xiaolangdi Reservoir of the Yellow River was impounded, the riverbed of the Huayuankou Reach was cut down by 3.44 m. However, coarsening of the riverbed promoted the development of larger sand dunes and thus increased the river resistance, resulting in an increase in the flood level when the discharge was greater than 6,100 m3/s (Ma et al. 2022). Therefore, it is necessary to study the mechanism of differential changes in water levels in large dammed rivers.

The Yangtze River is the busiest and largest navigable river in the world. Changes in low water levels determine channel navigation conditions and water access safety, and changes in flood water levels are the focus of flood control (Li et al. 2019; Hu et al. 2022b). The Three Gorges Dam (TGD; 30°44′18″N, 111°16′29″E), the world's most expansive dam, is strategically constructed within the upper reaches of the Yangtze River (Hu et al. 2023) and has been in operation for 20 years since 2003 (Figure 1). The Middle Yangtze River (MYR), which is close to the TGD, is experiencing dramatic fluctuations in terrain and water levels. At present, for changes in low water levels, existing studies agree that the continuous reduction in sediment inflow in the upper reaches leads to severe erosion in downstream rivers, and the water level at each hydrologic station under the same discharge decreases to different degrees (Yang et al. 2018; Zhang et al. 2024). With respect to changes in high water levels, existing studies generally state that there is no significant trend, mainly because the effect of increasing river resistance is close to the effect of erosion of the channel (Yang et al. 2018; Hu et al. 2022b, 2023). However, the water level change is not a discontinuous adjustment process. With increasing water discharge, the decreasing trend of the low water level must shift at a certain magnitude of discharge. Although relevant studies (Han et al. 2017; Yang et al. 2018; Chai et al. 2020) have mentioned the occurrence of this critical conversion discharge, they have rarely identified this discharge and have not provided a quantitative explanation for the cause. In addition, strong erosion caused by the construction of the TGD has developed in the lower part of the Jingjiang Reach (Dong et al. 2019). Therefore, the causes of water level changes in the Chenglingji–Jiujiang Reach (CJR) at different discharge levels need to be further clarified.
Figure 1

Map of the study area: the Yangtze River Basin in China and the CJR with locations of hydrometric stations.

Figure 1

Map of the study area: the Yangtze River Basin in China and the CJR with locations of hydrometric stations.

Close modal

The CJR is dominated by bifurcated rivers, and large cities such as Wuhan and Jiujiang are distributed within this region (Figure 1). With abundant hydrological and topographic data, changes in the water level have always been the focus of research. Since the operation of the TGD, flood control in the lower reaches of the dam has significantly improved; however, the main object for flood control of the TGD is the Jingjiang Reach, and the flood threat downstream of Chenglingji is still large due to the impacts of extreme rainfall, tributary flow, and climate change (Zhu et al. 2011; Xia & Chen 2021; Zhou et al. 2021). For example, the largest flood occurred in the Yangtze River since the operation of the TGD in 2020. The joint operation of reservoirs in the upper and middle reaches of the Yangtze River reduced the maximum water level from Yichang to Shashi by 3.0–3.6 m, but the Hankou station still experienced the fourth highest peak water level in history (Chen 2020). In addition, there are a large number of water-taking and water-regulating projects along the CJR, so it is urgent to study the characteristics and causes of water level variations after the operation of the TGD.

Thus, taking the CJR as an example, we analyzed water level change trend by using rich hydrological data via the Mann‒Kendall method, and computed geomorphic change and river resistance by using 406 cross-sectional profiles via cross-sectional topography method and one-dimensional hydrodynamic mathematical model. The objectives of this study are as follows: (i) to identify the characteristics of water level variations under different specific discharges and clarify the contributions of river erosion and silting, downstream water level adjustment, and change in resistance to these variations and (ii) to identify the critical conversion discharge of water level variations first and quantitatively explain the causes of critical conversion discharge. This study can provide a reference for predicting trends in water level adjustment and formulating countermeasures, such as navigation and flood control, after the construction of dams in different rivers around the world.

As the third longest river in the world, the Yangtze River, with a total length of ∼6,300 km, is generally divided into upper, middle, and lower reaches according to different hydrological characteristics and geographical settings (Lyu et al. 2020). The ∼521 km long reach studied herein, i.e., the CJR, is located between Chenglingji and Jiujiang in the lower part of the Middle Yangtze River, ∼446 km downstream of the TGD. The Han River, the largest tributary of the Yangtze River, flows in the middle of the CJR. The Dongting and Poyang floodplain-type lakes are at the head and end of the river, respectively (Figure 1), and now function as tributaries to the Yangtze River and create backwater effects in the river channel (Hu et al. 2022b).

As shown in Figure 1 and Table 1, the CJR covers 406 specified cross-sections surveyed by the Changjiang Water Resources Commission (CWRC), with an average spacing between two successive sections of approximately 2 km. The CJR is a typical braided channel with some central bars in the middle of the river. Most riverbanks in this reach consist of loose sediments that are often stratified, and the riverbed consists mainly of fine sand, with median diameters of 0.12–0.20 mm (CWRC 2021). The three hydrometric stations, Luoshan, Hankou, and Jiujiang, are located approximately 488, 697, and 967 km downstream of the TGD, respectively (Figure 1), and these hydrometric stations are representative of the hydrological conditions in the CJR. The CJR is classified into two subreaches, the Chenglingji-Hankou Reach (CHR) and the Hankou-Jiujiang Reach (HJR).

Table 1

Sources of measurements

Data typeLocationsPeriod of recordSources
Daily discharge Luoshan, Hankou, Jiujiang 1991–2022 CWRC 
Daily water level Luoshan, Hankou, Jiujiang 1991–2022 CWRC 
406 cross-section profiles per year Chenglingji–Jiujiang Reach 2004, 2013, 2020 CWRC 
Data typeLocationsPeriod of recordSources
Daily discharge Luoshan, Hankou, Jiujiang 1991–2022 CWRC 
Daily water level Luoshan, Hankou, Jiujiang 1991–2022 CWRC 
406 cross-section profiles per year Chenglingji–Jiujiang Reach 2004, 2013, 2020 CWRC 

Data sources

As shown in Table 1, hydrological data (i.e., daily mean discharges and water levels at the Luoshan, Hankou, and Jiujiang stations from 1991 to 2022) and topographical data (i.e., one-dimensional cross-sectional profiles at 406 specific locations for 2004, 2013 and 2020) were collected from the CWRC. Because the TGD began to fill with water in 2003, 2003 was taken as the boundary year to study the change in the water level before and after the completion of the reservoir. Because the TGD began to be affected by the storage of cascade reservoirs in 2013 (Dong et al. 2019), 2013 was taken as an important time node in the following analysis. Therefore, we divided the study period into 1991–2002, 2003–2012, and 2013–2022, which are called period 1, period 2, and period 3, respectively.

Mann–Kendall analysis of the trend in the water level change

Mann‒Kendall (M-K) analysis is an effective tool for analysis of the trends in hydrological elements, and its advantage is that samples do not need to follow a certain distribution (Hamed 2008; Li et al. 2022). M-K analysis was adopted to analyze the trends of the water level time series at the hydrological stations and to quantitatively determine the significance of the trends. The change in the trend is assessed by the statistical value Z. If Z > 0, the series shows an upward trend; otherwise, it shows a downward trend. Given the significance level α = 0.05 and corresponding Z1-α/2 = ±1.96, if |Z| > 1.96, the hydrological series is considered to have a significant trend. The index used to measure the magnitude of the trend was also the Kendall inclination β, which indicates the amount of change per unit time; β > 0 indicates an increase, β < 0 indicates a decrease, and the magnitude of the β value represents the average rate of change.

Cross-sectional topography method

The cross-sectional topography method is a widely used method for calculating the amount of erosion and deposition (Yuan et al. 2011). The amount of erosion and deposition under the corresponding water level can be determined by comparison with the storage capacity of each measured channel. First, the channel storage capacity of some measurements was calculated (Equation (1)), and the difference in volume between the two measurements was considered the amount of erosion and deposition (Equation (2)). The specific calculation equations are expressed as follows:
formula
(1)
formula
(2)
where V is the channel storage capacity, is the th cross-sectional area, is the distance between the th and th cross-sections, and is the amount of erosion and deposition. When , the riverbed experiences deposition; otherwise, erosion occurs.

One-dimensional hydrodynamic mathematical model

The theoretical basis and numerical discretization scheme of a one-dimensional hydrodynamic mathematical model are universally used (Dong et al. 2011), and the basic governing equations are Saint-Venant equations:
formula
(3)
formula
(4)
where K is the flow modulus, ; A is the water-crossing area, m2; Q is the discharge, m3/s; Z is the water level, m; n is the Manning roughness coefficient; R is the hydraulic radius, m; and v is the flow velocity, m/s.

Analysis of the factors influencing water level variations

Based on the principle of river dynamics, Manning's formula (Equation (5)) and the river water level gradient formula (Equation (6)) can be used to derive the expression of the factors influencing the water level (Equation (7)), which is widely used in water level research (Chai et al. 2020):
formula
(5)
formula
(6)
where Q is the discharge (m3/s); A is the cross-sectional area (m2); R is the hydraulic radius (m); J is the gradient of the river reach; n is the riverbed roughness; and are the water levels at the inlet and outlet sections (m), respectively; and L is the length of the river (m).
Assuming that the discharge at the inlet section remains unchanged during the entire period, i.e., Q=Q0 = Q1, we can derive Equation (7) from Equations (5) and (6) as follows:
formula
(7)
where the subscripts ‘0’ and ‘1’ represent the beginning and end of the period, respectively; the subscripts ‘in’ and ‘out’ represent the inlet and outlet sections, respectively; is the initial gradient of the reach; is the water level change in the inlet section during the period; is the water level change in the outlet section during the period; reflects the change in channel erosion and deposition; and reflects the change in river resistance.

We can conclude from Equation (7) that the variation in the water level is mainly the result of the combined effect of channel erosion and deposition, downstream water level variation, and resistance change. These three aspects are also the focus of the following analysis.

Selection of specific discharge criteria

To explore the trends in the variations at different water levels and identify the critical conversion discharge, representative discharges need to be selected at each hydrological station. The specific discharges were selected based on the following principles: (a) discharges must cover both flood and dry events; and (b) long-term time series of water levels can be obtained (Yang et al. 2018; Chai et al. 2020; Hu et al. 2022b). According to these principles and relevant references (Zhang et al. 2020), five specific discharge events were selected as the characteristic discharge events: low, low–medium, high–medium, bankfull, and flood (Table 2).

Table 2

Specific discharges at the Luoshan, Hankou, and Jiujiang stations in the CJR

DischargeLuoshanHankouJiujiang
Low discharge (m3/s) 10,000 12,000 12,000 
Low–medium discharge (m3/s) 20,000 20,000 20,000 
High–medium discharge (m3/s) 30,000 30,000 30,000 
Bankfull discharge (m3/s) 35,000 35,000 35,000 
Flood discharge (m3/s) 40,000 45,000 45,000 
DischargeLuoshanHankouJiujiang
Low discharge (m3/s) 10,000 12,000 12,000 
Low–medium discharge (m3/s) 20,000 20,000 20,000 
High–medium discharge (m3/s) 30,000 30,000 30,000 
Bankfull discharge (m3/s) 35,000 35,000 35,000 
Flood discharge (m3/s) 40,000 45,000 45,000 

Variations in water levels

Figure 2 shows the water level–discharge relationships from period 1 to period 3 at the Luoshan, Hankou, and Jiujiang stations. Under low discharge conditions, the range of fluctuation in the scattered points at each station decreased significantly after the operation of the TGD. This indicates that the water level decreased obviously under low discharge. With increasing discharge, the decreasing amplitude of the water level gradually decreased. Under high water discharge conditions, the range of fluctuation in the scatter points basically coincided with that before the operation of the TGD. It is worth noting that the range of fluctuation in the upper boundary of the Luoshan and Hankou stations exceeded the range of fluctuation before the operation of the TGD, implying that the floodwater levels of the Luoshan and Hankou stations increased obviously in some years.
Figure 2

Water level–discharge relationship in period 1 to period 3.

Figure 2

Water level–discharge relationship in period 1 to period 3.

Close modal

To analyze the trend of variation in the water level in the CJR before and after the operation of the TGD and to identify the critical conversion discharge, five specific discharges were selected for M-K analysis at the Luoshan, Hankou, and Jiujiang stations (Table 3). Since multiple data correspond to different water levels at the same discharge in each year and because the selected discharge data are missing in some years, to ensure the accuracy and completeness of the corresponding water level of the selected discharge in each year, the average water level within the range of each specific discharge ±5% was used in the analysis of the trend for each year. Due to missing water level and discharge data in some years, water level–discharge relationship interpolation was used to determine the data.

Table 3

Characteristic values of the water level at the Luoshan, Hankou, and Jiujiang stations

DischargeTimeLuoshan
Hankou
Jiujiang
ZβZβZβ
Low discharge 1991–2002 −0.481 −0.008 1.717 0.029 −2.541 −0.056 
2003–2022 −4.704 −0.096 −4.769 −0.076 −2.823 −0.036 
Low–medium discharge 1991–2002 1.030 0.010 1.580 0.019 −1.030 −0.050 
2003–2022 −4.250 −0.076 −3.536 −0.049 −1.233 −0.024 
High–medium discharge 1991–2002 −0.206 −0.018 0.481 0.016 −1.030 −0.055 
2003–2022 −2.563 −0.037 −0.876 −0.013 −0.032 −0.002 
Bankfull discharge 1991–2002 −0.343 −0.026 0.343 0.018 −1.854 −0.097 
2003–2022 0.389 0.003 0.357 0.005 0.941 0.015 
Flood discharge 1991–2002 0.000 0.003 0.069 0.013 −0.343 −0.042 
2003–2022 0.032 0.001 1.720 0.037 1.492 0.067 
DischargeTimeLuoshan
Hankou
Jiujiang
ZβZβZβ
Low discharge 1991–2002 −0.481 −0.008 1.717 0.029 −2.541 −0.056 
2003–2022 −4.704 −0.096 −4.769 −0.076 −2.823 −0.036 
Low–medium discharge 1991–2002 1.030 0.010 1.580 0.019 −1.030 −0.050 
2003–2022 −4.250 −0.076 −3.536 −0.049 −1.233 −0.024 
High–medium discharge 1991–2002 −0.206 −0.018 0.481 0.016 −1.030 −0.055 
2003–2022 −2.563 −0.037 −0.876 −0.013 −0.032 −0.002 
Bankfull discharge 1991–2002 −0.343 −0.026 0.343 0.018 −1.854 −0.097 
2003–2022 0.389 0.003 0.357 0.005 0.941 0.015 
Flood discharge 1991–2002 0.000 0.003 0.069 0.013 −0.343 −0.042 
2003–2022 0.032 0.001 1.720 0.037 1.492 0.067 

Table 3 shows that the water level at the different discharge levels exhibited different trends of change before and after the operation of the TGD. Before the operation of the TGD (1991–2002), except for the low discharge water level at the Jiujiang station, which showed a significant downward trend, the absolute Z value of the three stations at each discharge level was less than 1.96, indicating a nonsignificant trend. After the operation of the TGD (2003–2022), under low discharge, low–medium discharge, and high–medium discharge conditions at the Luoshan station, the Z value was less than −1.96, and the β value was negative, so the water level exhibited a significant downward trend. As the discharge increased, the Z and β values were close to 0 at bankfull discharge. It can be preliminarily determined that ∼35,000 m3/s is the critical transition point for water level change at the Luoshan station. Under the low discharge and low–medium discharge conditions at the Hankou station, the Z value was less than −1.96, and the β value was negative, so the water level exhibited a significant downward trend. As the discharge increased, the Z and β values were close to 0 at high–medium discharge. Similarly, we concluded that ∼30,000 m3/s is the critical transition point at which the water level changes at the Hankou station. The trend of the water level at the Jiujiang station was the same as that before the operation of the TGD, and the water level showed a significant downward trend under low discharge. As the discharge increased, the Z and β values were close to 0 at high–medium discharge. Therefore, ∼30,000 m3/s is also the critical transition point for water level change at the Jiujiang station. Under flood discharge, the Z value was greater than 0 but less than 1.96, and the β value was positive at all stations, so the water level showed a nonsignificant upward trend.

Impact of riverbed erosion

The amount of erosion in the low-flow, basic, bankfull and flood-flow channels in the CJR from 2003 to 2020 was calculated via the cross-sectional topography method (Figure 3). The ‘low-flow channel’, ‘basic channel’, ‘bankfull channel’ and ‘flood-flow channel’ refer to the channel below the corresponding water surface line when the discharge at the Yichang station (∼38 km downstream of the TGD) is 5,000, 10,000, 30,000, and 50,000 m3/s, respectively. The corresponding water levels at the Hankou station are 11.59, 17.26, 20.98, and 24.21 m (Zhang et al. 2024). In this study, the discharge is approximately equal to the total discharge of the channel under the four discharge conditions: low discharge, low–medium discharge, bankfull discharge, and high discharge. From 2003 to 2020, the cumulative erosion volumes of the low-flow, basic, bankfull, and flood-flow channels were 1,117.36, 1,127.36, 1,085.89, and 1,204.98 million m3, respectively. During period 3 (from 2013 to 2020), the cumulative erosion volumes were 830.21, 883.59, 894.78, and 959.67 million m3, respectively. Erosion mainly occurred in the low-flow channel, and the erosion intensity of the CJR increased significantly after storage occurred in the cascade reservoirs in 2013. The strengths of erosion in the low-flow channel in the CHR and the HJR increased from 25,300 to 38,700 m3/(km· year) during period 2 to 279,000 and 312,000 m3/(km· year) during period 3, and the whole reach changed from a state of weak erosion to a state of strong erosion.
Figure 3

Variation in the cumulative erosion and deposition strength over time.

Figure 3

Variation in the cumulative erosion and deposition strength over time.

Close modal
Six typical section profiles were selected to demonstrate the evolution of river channels (Figure 4). From 2003 to 2020, the low-flow channel significantly deepened, whereas the riverbed above the bankfull river channel hardly changed, which further indicated that riverbed erosion was an important reason for the decrease in the water level under low and medium discharge after the operation of the TGD.
Figure 4

Changes in typical section profiles in the CJR.

Figure 4

Changes in typical section profiles in the CJR.

Close modal
Changes in the longitudinal profiles of the thalwegs in the CHR and the HJR from 2003 to 2020 are shown in Figure 5. Since the operation of the TGD, fluctuations in the thalweg profiles have increased, indicating obvious erosion of the channel (resulting in a decrease in the water level) and likely enhanced morphological resistance (resulting in an increase in the water level).
Figure 5

Changes in the longitudinal profiles of the thalwegs in the CHR and HJR populations.

Figure 5

Changes in the longitudinal profiles of the thalwegs in the CHR and HJR populations.

Close modal

Impact of the downstream water level

The years 2004 and 2020 were selected as the initial and current representative years after the operation of the TGD, respectively. As shown in Table 4, the water level‒discharge relationships at the Luoshan, Hankou, and Jiujiang stations were fit to calculate the water level under a specific discharge. Under low discharge conditions, the water levels at the Luoshan, Hankou, and Jiujiang stations all decreased significantly (−1.11, −1.08, and −0.58 m, respectively), implying that the decrease in the water level at downstream hydrometric stations contributed to the decrease in water level at upstream hydrometric stations. Then, with increasing discharge, the influence of the downstream water level gradually weakened. Under flood discharge conditions, the water levels at the Luoshan, Hankou, and Jiujiang stations all obviously increased (0.45, 1.15, and 1.13 m), indicating that the increase at the downstream hydrometric station contributed to the increase in the water level at the upstream hydrometric station. The specific impact of the water level variation in the downstream hydrometric station under different specific discharge conditions on the water level of the upstream hydrometric station is specifically analyzed in Section 4.5.

Table 4

Water level variations at the Luoshan, Hankou, and Jiujiang stations under five discharge scenarios

DischargeWater level at Luoshan (m)
Water level at Hankou (m)
Water level at Jiujiang (m)
20202004Difference20202004Difference20202004Difference
Low discharge 18.50 19.61 −1.11 12.7 13.78 −1.08 7.16 7.74 −0.58 
Low–medium discharge 21.96 22.87 −0.91 16.27 16.93 −0.66 10.68 10.85 −0.17 
High–medium discharge 25.41 25.88 −0.47 19.78 19.82 −0.04 13.8 13.68 0.12 
Bankfull discharge 26.85 26.94 −0.09 21.14 20.92 0.21 14.97 14.65 0.32 
Flood discharge 28.15 27.70 0.45 23.49 22.34 1.15 16.8 15.67 1.13 
DischargeWater level at Luoshan (m)
Water level at Hankou (m)
Water level at Jiujiang (m)
20202004Difference20202004Difference20202004Difference
Low discharge 18.50 19.61 −1.11 12.7 13.78 −1.08 7.16 7.74 −0.58 
Low–medium discharge 21.96 22.87 −0.91 16.27 16.93 −0.66 10.68 10.85 −0.17 
High–medium discharge 25.41 25.88 −0.47 19.78 19.82 −0.04 13.8 13.68 0.12 
Bankfull discharge 26.85 26.94 −0.09 21.14 20.92 0.21 14.97 14.65 0.32 
Flood discharge 28.15 27.70 0.45 23.49 22.34 1.15 16.8 15.67 1.13 

Note: Difference means the value of water level at 2020 minus the value of water level at 2004.

Impact of river resistance

Using the one-dimensional hydrodynamic model, Manning's roughness coefficient (n) of the river under five specific discharge levels was estimated based on the 406 observed fixed section profiles and the measured hydrological data at the Luoshan, Hankou, and Jiujiang stations in 2004, 2013, and 2020. As shown in Table 5, taking the increase in the value/absolute value of the roughness coefficient as a proportion of the change, the roughness changed little at each discharge level before 2013 and decreased slightly (−2.8 to −4.08%) in the CHR, while the roughness increased slightly (0–7.62%) in the HJR. After 2013, the roughness at each discharge level increased significantly, and the roughness in the CHR clearly increased (by more than 7%) after the discharge reached bankfull discharge. The roughness in the HJR increased more obviously (close to 20%) after the discharge reached the high–medium discharge. In general, the increase in the amplitude of roughness in the HJR was greater than that in the CHR.

Table 5

Channel roughness n under different discharges from the CHR and HJR stations in 2004, 2013 and 2020

DischargeCHR
HJR
200420132020200420132020
Low discharge 0.026 0.025 0.027 0.022 0.024 0.027 
Low–medium discharge 0.026 0.025 0.027 0.022 0.022 0.025 
High–medium discharge 0.025 0.024 0.026 0.022 0.023 0.026 
Bankfull discharge 0.023 0.022 0.025 0.021 0.022 0.025 
Flood discharge 0.021 0.02 0.023 0.019 0.020 0.023 
DischargeCHR
HJR
200420132020200420132020
Low discharge 0.026 0.025 0.027 0.022 0.024 0.027 
Low–medium discharge 0.026 0.025 0.027 0.022 0.022 0.025 
High–medium discharge 0.025 0.024 0.026 0.022 0.023 0.026 
Bankfull discharge 0.023 0.022 0.025 0.021 0.022 0.025 
Flood discharge 0.021 0.02 0.023 0.019 0.020 0.023 

Main factors controlling water level variations at different discharge levels

To further define the main factors controlling water level variations at different discharge levels, the measured terrain data of the CJR, the downstream water level (water level at the Jiujiang station) and the calculated channel roughness (Table 5) in 2004 and 2020 were input into the one-dimensional hydrodynamic model based on the principles of control variables. The effects of channel erosion and deposition, downstream water level, and channel roughness adjustment on water levels at the Luoshan and Hankou stations under five discharge scenarios were calculated. Using the roughness of the CJR and downstream water level of the Jiujiang station in 2004, the impact of terrain adjustment on the water level (Ht) was obtained based on the difference between the water level under the measured terrain in 2004 and the water level under the measured terrain in 2020. Using the roughness and measured terrain of the CJR in 2004, the impact of downstream water level change on the water level (Hw) was obtained based on the difference between the downstream water level at the Jiujiang station in 2004 and the downstream water level at the Jiujiang station in 2020. Using the measured terrain of the CJR and downstream water level of the Jiujiang station in 2004, the impact of roughness change on the water level (Hn) was obtained based on the difference between the water level at the roughness value in 2004 and the water level at the roughness value in 2020. The specific results are shown in Figure 6.
Figure 6

Effects of the factors influencing the change in the water level under five discharge scenarios from 2004 to 2020. H is the measured change in the water level. H1 is the influence of the linear superposition of three factors on the water level. Ht is the impact of terrain adjustment on the water level. Hw is the impact of downstream water level change on the water level. Hn is the impact of roughness change on the water level.

Figure 6

Effects of the factors influencing the change in the water level under five discharge scenarios from 2004 to 2020. H is the measured change in the water level. H1 is the influence of the linear superposition of three factors on the water level. Ht is the impact of terrain adjustment on the water level. Hw is the impact of downstream water level change on the water level. Hn is the impact of roughness change on the water level.

Close modal

Figure 6 clearly shows the causes of the water level variations at the different discharge levels. From 2004 to 2020, the measured water level changes under the five specific discharges (−1.11, −0.91, −0.47, −0.09, and 0.45 m at the Luoshan station, and −1.08, −0.66, −0.04, 0.21, and 1.15 m at the Hankou station; Table 5) were approximately equal to the linear superposition values of three factors for the water level (−0.99, −0.85, −0.41, −0.06, and 0.47 m at the Luoshan station, and −1.02, −0.63, −0.03, 0.31, and 1.07 m at the Hankou station; Figure 5). This further verifies the conclusion of Subsection 3.4, namely, that the change in the water level is mainly the result of the comprehensive action of channel erosion and deposition, downstream water level change, and resistance change. However, the weight of influence of each factor was different under different discharge levels.

Under low discharge conditions, the influence values of topographic adjustment on the water level at the Luoshan station and the Hankou station were −1.17 and −1.83 m, respectively; the influence values of downstream water level change on the water level at the Luoshan station and the Hankou station were −0.06 and −0.05 m, respectively; and the influence values of comprehensive roughness change on the water level at the Luoshan station and the Hankou station were 0.25 and 0.92 m, respectively. Therefore, the main factor influencing the water level reduction at the Luoshan station and the Hankou station was channel erosion.

Under low–medium discharge conditions, the influence values of topographic adjustment on the water level at the Luoshan station and the Hankou station were −0.89 and −1.35 m, the influence values of downstream water level change on the water level at the Luoshan station and the Hankou station were −0.14 and −0.03 m, and the influence values of comprehensive roughness change on the water level at the Luoshan station and the Hankou station were 0.18 and 0.63 m, respectively. The main factor influencing the water level reduction at the Luoshan station and the Hankou station was channel erosion.

Under high–medium discharge conditions, the influence values of topographic adjustment on the water level at the Luoshan station and the Hankou station were −0.77 and −1.21 m, the influence values of downstream water level change on the water level at the Luoshan station and the Hankou station were 0.02 and 0.09 m, and the influence values of comprehensive roughness change on the water level at the Luoshan station and the Hankou station were 0.34 and 1.15 m, respectively. The main factor influencing the water level reduction at the Luoshan station was channel erosion, and the main reason for the lack of significant change in the water level at the Hankou station was that increasing resistance nearly counteracted the effect of channel erosion. In addition, the measured water level change and linear superposition effect of the three factors on the water level were close to 0 at the Hankou station. This further indicates that the critical conversion discharge of the water level change in the HJR was approximately 30,000 m3/s.

Under bankfull discharge, the influence values of topographic adjustment on the water level at the Luoshan station and the Hankou station were −0.74 and −1.17 m, the influence values of downstream water level change on the water level at the Luoshan station and the Hankou station were 0.1 and 0.09 m, and the influence values of comprehensive roughness change on the water level at the Luoshan station and the Hankou station were 0.58 and 1.39 m, respectively. The main cause of the nonsignificant variations in the water level at the Luoshan station was that increasing resistance nearly counteracted the effect of channel erosion. At the Hankou station, the main factor driving the increase in water level was that the effect of increasing resistance surpasses the effect of channel erosion. In addition, the measured water level change and linear superposition effect of the three factors on the water level were close to 0 at the Luoshan station. This further indicates that the critical conversion discharge of the water level change in the CHR was approximately 35,000 m3/s.

Under flood discharge, the influence values of topographic adjustment on the water level at the Luoshan station and the Hankou station were −0.67 and −1.19 m, the influence values of downstream water level change on the water level at the Luoshan station and the Hankou station were 0.29 and 0.33 m, and the influence values of comprehensive roughness change on the water level at the Luoshan station and the Hankou station were 0.85 and 1.72 m, respectively. The main reason for the increase in the water level at the Luoshan and Hankou stations was that the effect of increasing resistance was greater than that of channel erosion, and the increase in the downstream water level contributed to the increase in the water level.

A decrease in the downstream water level can be transmitted upstream to cause a decrease in the upstream water level. On the other hand, the increase in the downstream water level can increase the upstream water level. The effect of water level transmission varies greatly in different rivers. For the CJR, Section 4.5 quantifies the amplitude of the influence of downstream water level changes on the upstream water level at different discharge levels, but the underlying mechanism needs to be further clarified. Moreover, although the critical conversion discharge has been identified, why it exists and why the critical conversion discharge differs between the CHR and HJR remain to be determined.

Mechanism causing the influence of downstream water level change on upstream water level change

As shown in Table 4 and Figure 6, under low discharge conditions, the water level at the Hankou station decreased by 1.08 m, resulting in a decrease in the water level at the Luoshan station of 0.06 m, and the water level at the Jiujiang station decreased by 0.58 m, resulting in a decrease in the water level at the Hankou station of 0.05 m. In contrast, under flood discharge conditions, the water level at the Hankou station increased by 1.15 m, resulting in an increase in the water level at the Luoshan station of 0.29 m, and the water level at the Jiujiang station increased by 1.13 m, resulting in an increase in the water level at the Hankou station of 0.33 m. This shows that under low discharge, the water level change at the downstream hydrometric station has a small influence on the upstream hydrometric station. In contrast, under flood discharge, the water level change at the downstream hydrometric station has a greater influence on the upstream hydrometric station. To explain the cause of this phenomenon, we conducted the following numerical sensitivity test.

Using the measured topographies of the CHR and HJR in 2004, the thalweg and water surface lines under low discharge and flood discharge conditions were counted, and the water surface lines when the downstream hydrometric stations decreased by 1 m were compared (Figure 7). The thalweg fluctuates greatly. Under low discharge, there are several obvious drops that form ‘overwater weirs’, whose throat effect blocks or weakens the propagation of the downstream decreasing water level to the upper reaches, while the effect is not obvious under flood discharge. Therefore, the influence of downstream water level changes on the upper water level under low discharge conditions is less than that under flood discharge conditions. One situation that deserves particular attention is large flood caused by downstream tributaries, whereby the reservoir cannot regulate. For example, in the summer of 2020, the middle and lower Yangtze River, China, experienced an unprecedented monsoon season, whereby the biggest flood since the operation of the TGD originated from tributaries (Ma et al. 2022). A similar scenario occurred along the lower Yellow River in the summer of 2021, whereby an unforeseen flood formed from the tributaries downstream of the Xiaolangdi Dam due to extreme local precipitation (historically, the second largest) (Ma et al. 2022). The contribution of tributary water input to flood-stage amplification, and associated risk factors, requires further studies.
Figure 7

Comparison of water surface lines along the reach at low and flood characteristic discharges.

Figure 7

Comparison of water surface lines along the reach at low and flood characteristic discharges.

Close modal

Critical conversion discharge of water level change

Critical conversion discharge occurs widely in the lower reaches of dammed rivers. For example, after the construction of the Xiaolangdi Reservoir in the Yellow River in China, bed incision was accompanied by sediment coarsening, which facilitated the development of large dunes that increased flow resistance and reduced velocity relative to pre-dam conditions, resulting in the amplification of the downstream flood stage when the discharge was greater than 6,100 m3/s (Ma et al. 2022). Critical conversion discharge also occurs in the Yangtze River, and we propose three main reasons for the occurrence of critical conversion discharge. First, the water level at the critical conversion discharge was relatively high and close to the bankfull water level, and the channel shape was mainly a ‘V’ or ‘U’, and the channel width was significantly enlarged above this discharge level (Figure 4). Moreover, the extent of section expansion was mainly concentrated in the low water channel, and the water level decrease caused by erosion significantly decreased near this discharge. Second, reductions in the number of days of high flows caused by the TGD contributed significantly to the downstream growth of floodplain vegetation, and vegetation expansion increased floodplain resistance, which is thought to be the main factor driving the observed increase in water levels during high-flow inundation (stages at a given large discharge) in the MYR (Hu et al. 2022a, 2022b, 2023). Third, in recent years, the number of wading projects, such as bridges, wharves, and regulation projects, has increased intensively, and the pile foundations of wading projects have been mainly concentrated above low water channels. This increases the local resistance of the riverbed and the morphological resistance, and causes the water level and velocity field to experience superimposed impacts (Zhang et al. 2011; Yang et al. 2018).

As shown in Table 5, the river resistance of the CHR increased significantly when the discharge reached bankfull discharge, while the river resistance of the HJR increased significantly when the discharge reached high–medium discharge. This is why the critical conversion discharge of the HJR was lower than that of the CHR. We analyzed this difference in the composition of the river resistance. River resistance is mainly composed of grain resistance, dune resistance, bar resistance, morphological resistance, and additional resistance (Qian & Wan 1983). For the large alluvial river in the middle reaches of the Yangtze River, the bar resistance is mainly related to vegetation development, the morphological resistance is closely related to the plane shape and section shape of the channel, and the additional resistance is closely related to the density of wading projects, such as waterway regulation projects, bridges, and wharfs. At low water levels, the river resistance is mainly grain resistance, dune resistance, and morphological resistance (Hu et al. 2022a, 2024). With increasing discharge and water level, the influence of the bar resistance and additional resistance becomes increasingly serious.

Since the operation of the TGD, the grain resistance, dune resistance, bar resistance, morphological resistance, and additional resistance of the CHR and HJR have increased to some extent, which is the most important reason for the occurrence of critical conversion discharge. Specifically, from period 2 to period 3, the grain resistance of the CHR and HJR increased by 67 and 78%, the dune resistance increased by 2 and 7%, and the bar resistance increased by 14 and 2%, respectively (Hu et al. 2022a, 2022b). For the morphological resistance, the fluctuations in the thalweg profiles in the CHR and HJR increased (Figure 5), and the section profiles showed that the riverbed became increasingly narrower (Figure 4), which may have led to an increase in the morphological resistance. For additional resistance, the increases in the wading project density in the CHR and the HJR are quite different. Since the operation of the TGD, a total of 20 waterway regulation projects have been implemented in the CJR, including six in the CHR and 14 in the HJR. These projects mainly protect shorelines, side beaches, and central bars. They are good for navigation but increase river resistance. Since the operation of the TGD, a total of 17 bridges have been built in the CJR, including six in the CHR and 11 in the HJR. Bridge piers are generally located on the beach (Figure 8), and when the flow exceeds the medium discharge, the water-blocking rate (generally 1–5%) increases significantly (Chen et al. 2012), and the water-blocking rate increases with the increasing flowrate (Xu et al. 2010). Moreover, the HJR is located in the lower reaches of the CHR and has more shoreline development and utilization areas (CWRC 2016). From Google Maps in 2022, the two wharf groups with the highest densities in the CHR and HJR were selected for comparison. Figure 9 shows that the wharf density of the HJR is significantly greater than that of the CHR, and the water-blocking rate of the wharves also increases significantly after the flow exceeds the medium discharge. These wharf projects are also important causes of the increase in morphological resistance. Therefore, the increase in additional resistance in the HJR is more obvious. The impact of these wading projects on resistance requires further investigation and research. Moreover, in future wading project planning, the encroachment of river channels should be minimized, and the engineering density should be reduced to reduce the adverse impact on flood control.
Figure 8

Generalized bridge pier diagram.

Figure 8

Generalized bridge pier diagram.

Close modal
Figure 9

(a) Wharf groups in the CHR. (b) Wharf groups in the HJR.

Figure 9

(a) Wharf groups in the CHR. (b) Wharf groups in the HJR.

Close modal

Other causes of water level variations

The drainage capacity along the Yangtze River has significantly increased, which is also one of the important reasons for the increase in water level under the same discharge conditions. With the development of society and the economy and the acceleration of urbanization, the drainage capacity of the Yangtze River (especially in cities) has significantly increased. During heavy rainfall, rain-floodwater can be quickly pumped directly into the Yangtze River. Although the urban waterlogging situation has obviously improved, rivers and lakes have also filled rapidly, so the water level has risen sharply. In the middle and lower reaches of the Yangtze River, 55 medium-sized and above-scale pumping stations have an installed capacity of 318,000 kW, and the total drainage capacity of 57 culverts (with a designed flow of 50 m3/s or more) reaches 20,364 m3/s. In Hubei Province, which lies in the MYR, for example, 41 new pump (slug) stations have been built since 2016, increasing drainage discharge by 1,823 m3/s, and Wuhan's drainage capacity has increased from 980 to 1,960 m3/s. The waterlogging situation in the urban areas along the Yangtze River in 2020 was significantly better than that in 2016, but the floodwater level in the Yangtze River was also significantly greater than that in 2016 and was close to the level in 1998 (Chen 2020; Xia & Chen 2021). Therefore, according to the level of economic and social development and the importance of protection objects, urban and rural flood control and drainage standards should be scientifically established; the upstream and downstream regions of the river basin and the relationships between the left and right banks should be coordinated; the unified operation of large reservoirs, large sluices, and large pumping stations should be subordinated to overall interests; the comprehensive goal of control should be minimum comprehensive loss; and the relationships between external floods and waterlogging should be balanced.

It should be noted that increasing temperatures and human interventions have added stress to the region's hydrological sensitivity and have increased the risk of major flood events (Magnan et al. 2021; Shukla & Sen 2021; Rahmani & Fattahi 2024). Moreover, the strong rainstorm cycle in China has good consistency with the El Nino cycle (Kong 2020; Emberson et al. 2021; Zhou et al. 2021). It is necessary to develop better flood and drought risk management practices suitable for climate change and increased human activities.

We summarize our findings hereinafter. Decrease of the water level in the CJR has developed due to erosion of the channel bed downstream of the TGD. Increasing of the river resistance amplifies the water depth for a given water discharge. Especially, the river resistance increased by 7–20% from 2013 to 2020 when the discharge exceeded the bankfull discharge. Under the combined action of channel erosion and increased river resistance, the decreasing trend of the low water level shifts at a certain magnitude of discharge. Through our research, the critical conversion discharges of the water level changes in the CHR and the HJR are approximately 35,000 and 30,000 m3/s, respectively. Furthermore, we have clarified the underlying mechanism of why the amplitude of the influence of downstream water level changes on the upstream water level at different discharge levels is different. There are a number of obvious drops in the channel of the CJR, which form ‘overwater weirs’. Under low discharge, the throat effect blocks or weakens the propagation of the decreasing water level downstream to the upper reaches, while the effect is not obvious under flood discharge conditions. Therefore, the obvious increase in the floodwater level in individual years was due to the more obvious increase in the downstream water level.

In this article, the M-K method is used to analyze the trend of the water level variation in the CJR before and after the operation of the TGD. The influence of channel erosion and deposition, downstream water level adjustment, and river resistance change on water level change at different discharge levels is determined by using the separate variable method and a one-dimensional hydrodynamic model. Then, the critical conversion discharge of the water level change is identified first, and its causes are explained. The main conclusions are as follows:

  • (1) Before and after the operation of the TGD, the water level trend of the variation in the CJR exhibited different characteristics. Before the operation of the TGD, except for the low discharge water level at the Jiujiang station, which exhibited a significant decreasing trend, the water level at all three stations exhibited a nonsignificant trend. After the operation of the TGD, the trend of the water level at the Jiujiang station was the same as before. The water level at the Luoshan station at low, low–medium, and high–medium discharge levels exhibited a significant decreasing trend. The water level at the Hankou station at low and low–medium discharge levels exhibited a significant decreasing trend. When the discharge exceeded the bankfull discharge, the water levels at the Hankou and Luoshan stations showed a nonsignificant increasing trend. The floodwater level increased obviously in some years.

  • (2) The critical conversion discharges of the water level changes in the CHR and the HJR are approximately 35,000 and 30,000 m3/s, respectively. The significant decrease in the water level below the critical conversion discharge is mainly the result of the combined effect of channel erosion and increased resistance, and the dominant effect is channel erosion. The water level near the critical conversion discharge does not change significantly because the effect of increased resistance nearly offsets the influence of channel erosion, and the nonsignificant increase in the water level above the critical conversion discharge occurs because of the greater increase in resistance.

  • (3) There are a number of obvious drops in the channel of the CJR, which form ‘overwater weirs’. Under low discharge, the throat effect blocks or weakens the propagation of the decreasing water level downstream to the upper reaches, while the effect is not obvious under flood discharge conditions. Therefore, the obvious increase in the floodwater level in individual years was due to the more obvious increase in the downstream water level. The contribution of tributary water input to flood-stage amplification and the associated risk factors require further studies.

  • (4) More small flood discharges with higher water levels may become the new normal. In the face of great changes in flood control, we should provide enough storage space for floods and change from flood-fighting and emergency rescue modes to flood-related scientific management modes. On the other hand, overall planning for river basins is necessary to coordinate the flood control of major rivers and drainage along rivers and maintain peaceful, green, beautiful, and harmonious access to the Yangtze River.

This work was supported by the Yangtze River Water Science Research joint Fund of China (No. U2340217) and the Youth Project of National Natural Science Foundation China (No. 42301018). We would like to thank the Changjiang Water Resources Commission for providing hydrological and morphological data. We are grateful to the editor and manuscript reviewers for providing valuable comments on the revision of this paper. We would also like to thank the authors mentioned in all the references.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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